(1) Field of the Invention
The present invention relates to composite insulators each formed by molding an elastic insulating material around an insulating rod having an end fitting connected to each of opposite ends thereof. The invention also relates to a process for producing the same.
(2) Related Art Statement
Recently, many composite insulators having light weight and high strength have been used. As shown in FIG. 3, the composite insulator includes an insulating rod 51 made of glass fiber-reinforced plastic (FRP) or the like, end fittings 54 crimped to opposite end portions of the insulating rod 51, respectively, and an elastic insulating material 53 molded around the outer periphery of the insulating rod 51. A plurality of shed portions 52a are integrally formed with sheath portions 52b from the insulating material 53. Each of the end fitting grasps an end portion of the elastic insulating material 53.
However, there is a problem when such a composite insulator is used in an existing power transmission system in which the entire length of the insulator is preliminarily determined, for example, a feeding system for railroads. That is, the entire length of the insulator to be used in the existing electric power transmission wire system is preliminarily determined, and if the above composite insulator is designed to have the thus preliminarily determined entire length, the composite insulator cannot satisfy an electric characteristic required as a reference value (Lightning impulse withstand voltage, e.g., 320 kV).
The reason why the electric characteristic of the composite insulator does not satisfy the reference value is that since the effective insulating length between the end fittings 54 is narrowed by themselves, an insulating effective insulating length cannot be sufficiently ensured. The effective insulating length between the end fittings 54 may be increased by increasing the entire length of the insulating rod 51 and the molded elastic insulating material 53. The term "effective insulating length" used therein means a longitudinal space between both the end fitting. However, in this case, since the entire length of the rod 51, that is, the entire length of the composite insulator consequently increases, such an insulator cannot be used in the electric power transmission system in which the entire length of the insulator to be employed is preliminarily determined.
Further, in order to prevent invasion of water between the elastic insulating material 53 and the end fittings 54 in the composite insulator, it is necessary to provide a sealant 55 between the rod and the end fitting after the end fitting 54 is crimped around the rod 51. This is very inefficient from a manufacturing standpoint.